1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to equipment for optical wavelength division multiplexing (WDM) communication systems.
2. Description of the Related Art
An optical WDM communication system is designed to use optical signals having multiple carrier wavelengths (frequencies). Each carrier wavelength can be individually modulated with data and combined with other modulated wavelengths. The resulting optical WDM signal is transmitted over an optical communication link, e.g., an optical fiber, from a transmitter to a receiver, where the modulated wavelengths are separated and individually processed to recover the data. This ability of a WDM system to transmit many data streams in parallel advantageously enables the system to have a relatively large transmission capacity.
One type of modulation that is often used in WDM systems is phase-shift keying (PSK). More specifically, PSK modulation encodes data onto the carrier wavelength using the wavelength phase. For example, in its simplest (binary) form, PSK encodes digital “ones” and “zeros” as two respective phases that are 180 degrees apart. Other forms of PSK can similarly encode data using more than two phases.
One method of decoding a PSK-modulated signal at the receiver is based on homodyne detection, in which the received PSK-modulated signal is combined with a local oscillator (LO) signal. The intensities of the resulting interference signals are then measured to determine the modulation phase. It is well known in the art that PSK modulation combined with homodyne detection can advantageously provide a significant sensitivity improvement over other optical modulation-detection combinations.
One problem with the use of homodyne PSK is that the homodyne detection of each modulated signal utilizes a LO signal that is phase-locked to the carrier wavelength of that modulated signal. However, at the receiver, a phase-lock to the carrier wavelength is relatively difficult to achieve because the PSK causes pseudo-random phase variation in the received signal. Also, the task of providing multiple phase-locked LO signals for multiple wavelengths further complicates the use of homodyne PSK in prior-art WDM systems.